A conventional television transmitter system capable of broadcasting thirty one channels costs almost two million dollars to build in 1991. The high cost of this well known system is high because thirty one sets of parts must be provided, i.e., each channel requires an entire set of parts. More particularly, a single channel microwave transmitter receives as its input an audiovisual signal; the audio part of the signal is fed to an audio modulator, and the visual part thereof is fed to a video modulator. The modulated signals are then fed, respectively, to an audio microwave upconverter and a video microwave upconverter. The output signal of each upconverter is then fed to driver amplifiers and the output of those amplifiers is then fed to power amplifiers and the amplified signals are then combined in an audiovisual microwave combiner. The combined audiovisual signal is then sent to an antenna combiner for broadcasting. For a thirty one channel system, two expensive antennas and ten microwave combiners are required. A first antenna is fed by the output of a microwave combiner having four inputs, and the four inputs of the combiner are in turn fed by the outputs of four additional combiners, thereby providing a means of connecting sixteen odd-numbered channels to the first antenna without cross-coupling between the transmitters; cross-coupling would destroy the television pictures. The fifteen even-numbered channels are fed to the second antenna in a similar manner. Each of the thirty one channels includes a set of the above-described parts, i.e., each channel includes its own visual microwave upconverter and its own audio microwave upconverter, as aforesaid. Thus, there is much duplication of expensive equipment, but the prior art, when considered as a whole in accordance with the requirements of law, neither teaches nor suggests to those of ordinary skill in this art how such duplication of parts could be avoided.
It has long been the conventional wisdom that the aforesaid structure is required simply by the laws of physics. In the visual and aural microwave upconverters, a common local oscillator frequency is required. The output signal of a crystal oscillator is fed to a harmonic generator amplifier and through a three cavity filter which selects the desired harmonic, typically the 45th harmonic. An amplifier builds up the power of the selected harmonic for use as the local oscillator signal. The harmonic amplifier feeds a splitter that provides two local oscillator signals of equal power to the mixers in the visual and aural upconverters. The mixer output of the visual and aural signals are amplified, passed through a bandpass filter, amplified again and broadcast. Thus, a single channel requires a pair of mixers and associated amplifiers and filters.
The obvious solution to this problem, from the perspective of a layperson, would be to feed multiple channel signals to the upconverters so that individual transmitters would not be needed for each channel. Due to the very high cost of providing equipment dedicated to each channel contained within the source signal, this approach has been considered, but those skilled in the electrical arts need not build a prototype of such a multiple channel upconverter to demonstrate its lack of utility because it is well known that such an approach could not work. Specifically, microwave mixers are not ideal devices; they have poor power handling capability, and low local oscillator isolation. Accordingly, the local oscillator power applied to the local oscillator port leaks out of the input and output ports. Power leaking back to the source of the multi-channel signal intermodulation splatters the channels into new unwanted frequencies and such extra-band broadcasting can of course lead to loss of license. Thus, those skilled in the art have eschewed the idea of feeding a plurality of channels into a single television transmitter.